Infant formula does not provide this benefit, which helps protect infants from infections and illnesses, the Duke University Medical Center researchers said.

The team grew two strains of E. coli bacteria in samples of breast milk, infant formulas (both milk- and soy-based) and cow's milk. The E. coli strains are necessary early inhabitants of the gut and are beneficial cousins of the E. coli strains that cause food poisoning.

The bacteria began multiplying in all the samples, but there was an immediate difference in the way they grew. In breast milk, the bacteria stuck together to form biofilms, which are thin layers of bacteria that act as a shield against harmful microorganisms and infections.

Bacteria in the infant formula and cow's milk grew as individual organisms that did not form biofilms, according to the findings, published in the August issue of the journal Current Nutrition & Food Science.

"This study is the first we know of that examines the effects of infant nutrition on the way that bacteria grow, providing insight to the mechanisms underlying the benefits of breast feeding over formula feeding for newborns," study senior author William Parker, an associate professor of surgery, said in a Duke news release.

"Only breast milk appears to promote a healthy colonization of beneficial biofilms, and these insights suggest there may be potential approaches for developing substitutes that more closely mimic those benefits in cases where breast milk cannot be provided," he added.

Previous studies have shown that breast milk reduces infants' risk of diarrhea, influenza and respiratory infections and protects against later development of allergies, type 1 diabetes, multiple sclerosis and other illnesses.

"Knowing how breast milk conveys its benefits could help in the development of infant formulas that better mimic nature," Parker said. "This could have a long-lasting effect on the health of infants who, for many reasons, may not get mother's milk."

More information

The U.S. National Institute of Child Health and Human Development has more about breast-feeding.